#6 warn if redundant renderPassManager nodes found

[Andrew Chapman]

The plugin finds the renderPassManager node by name, so
if you're importing/merging scenes you might end up
with renderPassManager2, renderPassManager3, etc.

They don't cause trouble, but it might be good to at
least warn the user when it happens, and perhaps give
them the option of choosing one to use as the main one,
or even merging them together somehow.

This check can happen when opening the render pass
manager window. It should be possible to hook it into
scriptjobs that get called on open/import, but that
would complicate things.